1. Field of the Invention
The present invention relates to a multi-antenna and an electronic device used in wireless routers, mobile telephones, and the like.
2. Description of Related Art
The need for data communication for accessing the internet or the like using a mobile telephone is growing rapidly. Also, in recent years, household electric devices, other than mobile telephones, that have wireless communication systems have become more popular. In particular, devices provided with Bluetooth (trademark) or wireless LANs have become popular.
A diversity antenna has been used conventionally as a method for using a plurality of antennas in order to attain excellent communication sensitivity while conducting high speed transmission. The diversity antenna automatically selects a signal from an antenna among a plurality of antennas that has the greatest output. As a result, it became possible to conduct excellent communication compared to a case in which only one antenna is used, thus attaining a higher speed data transmission.
Recently, besides the diversity antenna, a modulation method known as an MIMO (multiple-input and multiple-output) communication system for conducting even faster communication has rapidly grown in use. The MIMO technique is provided with a plurality of antennas on both the transmitting side and the receiving side, and by attaining a transmission path that has been spatially multiplexed, the transmission capacity can be increased, thus increasing the communication speed. The MIMO technique is used in the following communication standards, for example.                For wireless LANs, the IEEE 802.11g standard has been established, and devices such as wireless LAN routers that follow this standard have become popular.        Mobile telephones that can conduct high speed data transmission according to the LTE (long term evolution) standard are starting to become popular.                    Up until the present, internet connection was done by constructing communication lines (electrical wires or optical cables) from a server to each household, company, or the like, but WiMAX (IEEE 802.16e) has been established as a standard for converting this to wireless. As a result, internet connection infrastructure can be provided with ease to regions and the like with low populations. Also, in recent years, WiMAX has started to be used with mobile devices.                        
As wireless techniques that use a plurality of antennas such as diversity antennas or MIMO become popular, antennas for mobile devices with a small size, maximum isolation properties, and high efficiency are desired.
Patent Document 1 discloses an antenna device that is provided with a monopole passive element on one surface of an insulating substrate, two monopole antenna elements provided on the other surface with a gap therebetween, and a ground pattern for each monopole between the antenna elements.
In this antenna device, the two antennas have opposite directionalities to each other by disposing the ground pattern between the two antenna elements, which improves isolation properties.
Patent Document 2 discloses a multi-antenna device provided with a passive element between two monopole antenna elements such that a current generated by capacitive coupling flows in the opposite direction to the current flowing to these elements.
This multi-antenna device has improved isolation by reducing mutual coupling to the element between the antenna elements by providing a passive element of the above configuration therebetween.